The invention relates to a method for internal high-pressure forming of a workpiece and to an apparatus for carrying out the method.
A method of the generic type and an apparatus of the generic type are known from DE 94 07 812.2 U1. Here, a hollow section forming the workpiece is inserted into an internal high-pressure forming die split in the direction of extension of the hollow section, after which the die is closed. The hollow section is then closed in a sealing manner at both ends by axial rams. The interior of the hollow section is filled via the axial rams. The axial rams have a fluid connection to a pressure generator. An internal high pressure within the hollow section is then applied by means of the pressure generator, thereby expanding the section until it comes to rest against the wall of the die cavity. The die cavity has a branch that leads radially away from the direction of extension of the hollow section and into which the material of the hollow section can be displaced by the imposition of the internal high pressure to form a neck. Here too, the material comes to rest against the wall of the branch. The process of expansion within the branch is stabilized by means of a sliding counter plug, which is guided displaceably in the branch and supports the neck at the end. To raise the failure limits as regards bursting during pure expansion, during which the length of the hollow section decreases, and thus obtain a longer length of expansion in the neck, an additional axial force, which is applied by moving the axial rams inwards, is used to feed additional material from the hollow section to the branching point, thereby at least partially compensating for the thinning of the material in the region of the branch that is responsible for bursting. Nevertheless, the friction that arises between the die and the hollow section when the material of the hollow section comes to rest against the wall of the cavity, specially in the branch, which increases as the internal high pressure increasesxe2x80x94even when the outside of the hollow section is provided with a lubricantxe2x80x94considerably restricts the formability of the material. This is very problematic, especially in the case of materials with poor formability.
The object on which the invention is based is to develop a method of the generic type and an apparatus of the generic type in such a way that process reliability during the internal high-pressure forming of workpieces is improved.
According to the invention, the object is achieved as regards the method by the features of Patent claim 1 and as regards the apparatus by the features of Patent claim 11. Owing to the excitation of vibration in one of the friction partners (workpiece, forming die), the friction of the workpiece on the wall of the cavity is reduced in between the maxima of the vibration since there the contact force of the workpiece on the forming die is lowered. This means that there is periodic partial relief of the normal contact stress between the workpiece and the die during the forming operation. In drastic cases, this can be such that locally the workpiece loses contact completely with the die for a brief period. However, the associated reduction in friction also means that resistance to the inflow or infeed of additional workpiece material towards the forming location also decreases, allowing more material to be moved to this location without failure occurring. Thinning of material in the forming zone is thus counteracted, it being possible to achieve a distribution of material that is more favourable for forming, in particular more uniform wall thickness distribution and/or to increase the degree of forming. In the case of a branch from the cavity for example, i.e. the formation of a neck, this can mean an increase in the length of extension. The limits of the method can thus be extended, e.g. as regards the production of secondary formed elements. Owing to the increased supply of additional material, it is furthermore also possible to form smaller radii on the workpiece in a reliable process without the occurrence of cracking. Overall, improved forming, even of workpieces made of materials with a low deformability, is achieved by means of the invention. By partial decoupling of the process parameters from the tribological conditions, higher process stability is achieved. For optimum design of the method to give the maximum utility for the respective forming task, the amplitude and frequency of the vibration should be adapted to take account of the material of the workpiece and degree of forming involved in the shape to be obtained, i.e. the geometry of the fully formed workpiece or other relevant process parameters. Moreover, the structural vibration of the friction partners leads to more uniform distribution of lubricants when using lubricants on the surface of the workpiece, leading to a further reduction in friction. Owing to the vibratory relative motion of the friction partners and the activation of the lubricant introduced, adhesive contact is to the greatest possible extent avoided while sliding friction is simultaneously reduced, and the transition from static to sliding friction (stick-slip effect) is considerably reduced. This makes it possible to select a lubricant for less demanding requirements and/or to apply less lubricant or even to dispense with lubricant while ensuring improved forming.